One type of semiconductor package is referred to as a BGA (ball grid array) package. A conventional BGA package includes a semiconductor die bonded to a substrate, and an encapsulant on the die. Typically, the substrate comprises an organic material, such as bismaleimide triazine (BT), an epoxy resin (e.g., “FR-4”) or a polyimide resin. The substrate also includes a pattern of conductors, such as copper traces formed directly on the substrate, or alternately on a flexible tape attached to the substrate.
One type of BGA package is known as a BOC (board-on-chip) package. With a BOC package, the substrate (i.e., the board) is bonded to the circuit side (face) of the die, and wire bonds are made between the die contacts on the substrate and the bond pads on the die. Another type of BGA package is known as a COB (chip-on-board) package. With a COB package, the die is back bonded to the substrate and wire bonded to the die contacts on the substrate, or alternately flip chip bonded directly to the die contacts on the substrate.
The substrate also includes external contacts in electrical communication with the conductors and the die contacts. Typically, the external contacts comprise solder balls arranged in a dense array, such as a ball grid array (BGA), or a fine ball grid array (FBGA).
Referring to FIGS. 1A–1C, a conventional BGA package 10 in a COB configuration is illustrated. The BGA package 10 includes a stacked pair of semiconductor dice 12, 14, each having a pattern of bond pads 16 on a circuit side thereof. The dice 12, 14 are sized, and bonded to one another, such that the bond pads 16 are exposed for wire bonding. The bond pads 16 are in electrical communication with the integrated circuits and semiconductor devices contained on the dice 12, 14. The BGA package 10 also includes a substrate 18 and adhesives layers 20 which bond the dice 12, 14 to one another and to the substrate 18. The substrate 18 includes die contacts 22 configured for wire bonding to the dice 12, 14. Specifically, wires 24 are bonded to the bond pad 16 on the dice 12, 14 and to the die contacts 22 on the substrate.
The BGA package 10 also includes a polymer tape 28 on the substrate 18, and an array of external contacts 26 on the polymer tape 28 in electrical communication with the die contacts 22 on the substrate 18. Conductors (not shown) on the polymer tape 28 and interlevel conductors (not shown) on the substrate 18 electrically connect the external contacts 26 to the die contacts 22. Typically, the polymer tape 28 includes a flexible polymer substrate, such as polyimide, on which required circuit patterns are formed.
The external contacts 26 comprise solder balls in a grid array, such as a ball grid array (BGA) or a fine ball grid array (FBGA). The external contacts 26 are bonded to ball bonding sites 30 on the polymer tape 28 using a bonding technique such as soldering, welding or brazing. In addition, a solder mask 34 includes openings 36 (FIG. 1C) for aligning the external contacts 26 for bonding, and for electrically insulating the external contacts 26 from one another.
The BGA package 10 also includes an encapsulant 32 that encapsulates the dice 12, 14, the wires 24, and the associated wire bonds on the bond pads 16 and on the die contacts 22. Typically, the encapsulant 32 comprises a Novolac based epoxy formed in a desired shape using a transfer molding process, and then cured using an oven.
One feature of this type of BGA package 10 is that due to their size, the external contacts 26 add considerably to the thickness T (FIG. 1A) of the BGA package 10. For example, conventional solder balls typically have a diameter of from about 0.012-inch (0.3-mm) to 0.030-inch (0.762-mm). The size of the external contacts 26 also limits the density or “packing fraction” of the external contacts 26. Similarly, due to their size, the spaces between the external contacts 26 are relatively small, so that the routing of the corresponding conductors on the polymer tape 28 in the spaces is restricted.
It would be advantageous to be able to fabricate external contacts that are smaller than conventional solder balls. This would decrease the height of the BGA package 10. In addition, smaller external contacts could be spaced further apart allowing the density of the conductor pattern on the polymer substrate 28 to increase. This in turn would decrease the peripheral outline, or “footprint”, of the BGA package 10.
Another problem with external contacts 26 in the form of conventional solder balls, is that the locations of the external contacts 26 may vary in the X-direction from a theoretical location (dotted lines), as indicated by ΔX in FIG. 1B, or in the y-direction as indicated by ΔY in FIG. 1B. These variations in the locations of the external contacts 26 may be due to mask alignment and art work errors introduced during fabrication of the bonding sites 30, and during fabrication of the solder mask 34.
Another problem with external contacts 26 in the form of solder balls is that the planarity of the external contacts 26 may vary. For example, as shown in FIG. 1C, the middle external contact 26 is offset from a theoretical plane P by a distance AZ. This planarity variation may be due to warpage of the polymer substrate 28 following the mold cure process for the encapsulant 32.
Yet another problem with external contacts 26 in the form of solder balls is illustrated in FIG. 1D. Typically the package 10 is mounted to a supporting substrate 38 (e.g., PCB), by reflowing the external contacts 26RF onto electrodes 40 on the supporting substrate 38. A solder mask 42 on the supporting substrate 38 facilitates the soldering process. However, the planarity of the external contacts 26 can adversely affect the planarity of the package 10 as indicated by the angle A of the package 10 relative to a theoretical package planarity PP (where PP is a plane generally parallel to the surface of the supporting substrate 38).
Another problem associated with external contacts 26 in the form of solder balls, is that poor quality solder joints between the external contacts 26 and the bonding sites 30 can cause opens and shorts, or can cause the external contacts 26 to separate from the bonding sites 30. Poor quality solder joints can also cause the external contacts 26 to separate from the bonding sites 30. In general, the quality of the solder joints is affected by the surface topography of the bonding sites 30 and by the thermal profile and cycling times of the bonding process. Similarly the quality of the solder joints between the reflowed external contacts 26RF (FIG. 1D) can cause opens and shorts, and planarity problems for the package 10.
In view of the foregoing improved semiconductor packages having better external contacts are needed in the art. The present invention is directed to an improved semiconductor package having multi layered external contacts, and optionally multi layered die contacts. The present invention is also directed to a method for fabricating the package, and to improved electronic assemblies incorporating the package.